Inductive power transfer relies on alternating power inductively coupled from conductors and received in a power receiver by inductive transfer from those conductors. The receipt of power may be augmented by field concentrating devices and/or by tuning the power receiver to the power frequency.
This specification relates to systems in which the receiver contains a circuit which is resonant at or near the frequency of the current in the conductors. Known circuits for inductive power transfer may produce substantial reactive power flow when the tuned circuit is detuned to reduce power flow.
1. The Problem
The problem of unwanted reactive power flow can be almost completely overcome using what are known as “decoupling” controllers, however the choice available to designers is very restrictive. These controllers act by disconnecting the tuned circuit from the load, that is by decoupling the load from the inductive power receiving circuit. The decoupling may occur at a slower rate than the power frequency, or it may occur at a faster rate than the power frequency. In either case the average power taken from the power source is reduced. The relationship between the time coupled and the time decoupled may vary, so that the controller effectively acts to partially variably decouple the load.
If a parallel circuit topology is used for the inductive power receiver, then the maximum load current is fixed and increases in power for short transient situations can only be achieved by increasing the output voltage. Since most equipment is voltage sensitive this is unacceptable. Conversely if the power receiving circuit is a series tuned circuit it has a fixed maximum output voltage but does allow increases in power by transient surge currents. Here the average output voltage may be set by the switch duty cycle but the current may then be allowed to increase transiently as required. However these transient surges correspond to high voltages across the pick-up coil and the tuning capacitor and these may be unacceptable.
2. Prior Art
Resonant inductive power transfer is known, see for instance:
U.S. Pat. No. 4,914,539 by Turner et al. describes an inductive power transfer system for supplying power to airline seats. The specification describes a resonant power receiver which is synchronously regulated by shorting across the resonant circuit for a portion of the positive-going cycles. The construction of this circuit is such that shorting across the resonant circuit for substantially less than quarter of a cycle per cycle varies the circuit reactance. The change in circuit reactance shifts the circuit resonance and hence reduces the available power out. However substantial reactive power flow still exists between the track conductors and the receiver and hence it should be noted that this is not decoupling of the output.
Publication WO 01/71882 describes a resonant circuit in which a synchronous switch shorts across a parallel resonant circuit for portions of leading half cycles of the power waveform. This circuit could induce dangerous conditions if the switch or the load failed in an open condition.
U.S. Pat. No. 5,293,308 describes a resonant circuit in which power flow from the track conductors to the receiver can be essentially completely decoupled. Here an asynchronous switch either shorts across a parallel resonant circuit or opens at least a portion of a series resonant circuit at any switching frequency required. In either case the power transferred to the resonant circuit can be reduced to near zero. Variation of the duty cycle of the switch can provide any required output power up to the maximum available. In the minimum power condition there is essentially no adverse reactive power flow.